This invention relates to a method of recovering alkali values from trona ore. In particular, this invention relates to a process for dissolving trona ore to recover separately sodium bicarbonate and sodium carbonate and produce dense soda ash.
Trona ore is a mineral that contains about 90-95% sodium sesquicarbonate (Na.sub.2 CO.sub.3 .multidot.NaHCO.sub.3 .multidot.2H.sub.2 O). A vast deposit of mineral trona is in southwestern Wyoming near Green River. By conservative estimates, this deposit contains about 75 billion metric tons of trona ore.
The sodium sesquicarbonate found in trona ore dissolves in water to yield approximately 5 parts by weight sodium carbonate (Na.sub.2 CO.sub.3) and 4 parts sodium bicarbonate (NaHCO.sub.3). To recover these valuable alkali products, the trona ore must be processed to remove insoluble materials and other impurities.
One such valuable alkali produced from trona is soda ash (a commercial grade of sodium carbonate). Soda ash is one of the largest volume alkali commodities made in the United States. In 1992, trona-based soda ash from Wyoming comprised about 90% of the total U.S. soda ash production. Soda ash finds major use in the glass-making industry and for the production of baking soda, detergents and paper products.
Typically, dense soda ash is produced from trona ore in a process known as the "monohydrate process", which consumes great quantities of water (a scarce and valuable resource in Wyoming) and energy. In that process, crushed trona ore is first calcined (i.e., heated) at a temperature between 125.degree. C. and 250.degree. C. to convert the sodium bicarbonate into sodium carbonate.
When the trona ore is calcined, the sodium sesquicarbonate in the trona ore breaks down into sodium carbonate, carbon dioxide and water. Also, calcination releases some of the organics associated with trona or trona shale.
The resulting sodium carbonate and the released organics are then dissolved in water. Any undissolved solids are then filtered and the solution is treated with activated carbon to remove some of the dissolved organics.
The filtered solution of sodium carbonate is fed to an evaporative crystallizer. Water is evaporated and the sodium carbonate forms into sodium carbonate monohydrate crystals (Na.sub.2 CO.sub.3 .multidot.H.sub.2 O). The crystals are removed and then calcined, or dried, to convert it to dense soda ash.
One of the perceived advantages of the monohydrate process is that calcined trona dissolves faster than raw trona. Another perceived advantage is that dissolved, calcined trona produces a more concentrated sodium carbonate solution of about 30%, while dissolved raw trona produces a solution having only about 16% sodium carbonate plus 10% sodium bicarbonate.
However, the monohydrate process has several disadvantages. Presently, the monohydrate process consumes considerable amounts of water. When the raw trona ore is calcined, the natural water content is evaporated. Yet after that water is evaporated off, more water must be added to dissolve the calcined trona.
In addition to consuming water, the monohydrate process consumes and wastes considerable amounts of energy. Calcining equipment, for example, has only about 50% energy efficiency. Much energy is also wasted in calcining simply to raise the temperature of the sodium carbonate in the trona up to the calcining temperature even though the sodium carbonate itself does not undergo conversion. Finally, a significant amount of energy is wasted during calcining in evaporating the naturally occurring water in the trona.
Another disadvantage of the monohydrate process is that the feed solution can only contain a maximum of about 30% sodium carbonate. The evaporative crystallizers could be more efficiently operated if a more concentrated feed solution were used.
Therefore, there is a need to provide a simple, energy-efficient and water-conserving process to recover alkali values from trona ore.